ADAPTIVE IMMUNITY STUDY GUIDE

CRITICAL CD MARKERS

• Purpose: Essential for identifying different immune cell types, crucial for exams.

CD Marker Table

• CD3

  • Cell Type: All T cells

  • Function/Notes: Part of TCR complex, signal transduction.

  • Note: Universal T cell marker.

• CD4

  • Cell Type: Helper T cells

  • Function/Notes: Co-receptor for MHC II; HIV receptor.

  • Note: HIV targets these cells.

• CD8

  • Cell Type: Cytotoxic T cells

  • Function/Notes: Co-receptor for MHC I; kills infected cells.

• CD10

  • Cell Type: Immature B cells

  • Function/Notes: Marker of early B cell development.

  • Note: Emphasized for importance in learning.

• CD19

  • Cell Type: All B cells

  • Function/Notes: Co-receptor, signal amplification.

  • Note: Most important B cell marker.

• CD20

  • Cell Type: Mature B cells

  • Function/Notes: Involves calcium signaling.

  • Note: Target of Rituximab.

• CD21 (CR2)

  • Cell Type: Mature B cells

  • Function/Notes: Complement receptor (binds C3d) and EBV receptor; enhances B cell activation.

• CD34

  • Cell Type: Hematopoietic stem cells, early B cells

  • Function/Notes: Stem cell marker; Used to isolate stem cells.

• CD28

  • Cell Type: T cells

  • Function/Notes: Costimulation receptor (binds CD80/CD86); facilitates signal 2 for T cell activation.

• CD40

  • Cell Type: B cells, APCs

  • Function/Notes: Binds CD40L on T cells; facilitates T cell help for B cells.

• CD16

  • Cell Type: NK cells, neutrophils

  • Function/Notes: Fc receptor (ADCC); NK cell marker.

• CD56

  • Cell Type: NK cells

  • Function/Notes: Adhesion; another NK cell marker.

FLASHCARD FORMAT

• Front: "Which CD marker is found on ALL B cells and is the most important B cell marker?"

  • Back: "CD19"

• Front: "CD10 is a marker of what stage of B cell development?"

  • Back: "Immature B cells"

MHC CLASS I vs MHC CLASS II

• Purpose: Differentiate between two types of MHCs; necessary for understanding antigen presentation.

Comparison Table

• Feature
  MHC CLASS I

  • Structure: Heavy chain (α) + β2-microglobulin

  • Peptide Binding Groove: Closed at both ends → binds 8-10 amino acids.

  • Expressed on: All nucleated cells (not RBCs).

  • Peptide Source: Endogenous (self or viral proteins from inside the cell).

  • Presents to: CD8+ T cells (cytotoxic T cells).

  • Function: Displays internal happenings (detects infected/abnormal cells).

  • Loading Location: Endoplasmic Reticulum (ER).

  • Key proteins: TAP (transporter), Tapasin, Calreticulin.

  MHC CLASS II

  • Structure: α chain + β chain (both transmembrane).

  • Peptide Binding Groove: Open at both ends → binds 13-25 amino acids.

  • Expressed on: APCs ONLY (dendritic cells, macrophages, B cells).

  • Peptide Source: Exogenous (from outside - phagocytosed materials).

  • Presents to: CD4+ T cells (helper T cells).

  • Function: Displays external antigens for recognition.

  • Loading Location: Endosome/lysosome.

  • Key Proteins: Invariant chain (Ii), HLA-DM.

Memory Tricks

• MHC I = 1 chain (plus β2m), interacts with CD8 (1 “8” in the name) and processes internal (endogenous) peptides.

• MHC II = 2 chains, interacts with CD4 (1 “4” in the name) and processes external (exogenous) peptides.

• Phrase: "All cells show ID" = All nucleated cells have MHC I, whereas only "Professional presenters" (APCs) have MHC II.

MHC CLASS I PEPTIDE LOADING PATHWAY

Process Flow (Endogenous Pathway)

1. Protein Degradation in Cytoplasm

   • Viral/self-proteins get tagged with ubiquitin for degradation.

   • Degradation by proteasome produces short peptides (8-10 amino acids).

2. Transport into ER

   • TAP (Transporter Associated with antigen Processing) moves peptides to the Endoplasmic Reticulum (ER).

   • TAP is a heterodimer (TAP1 + TAP2) acting like a doorway that excludes irregular sizes.

3. MHC I Assembly in ER

   • Constructed with heavy chain (α chain) + β2-microglobulin (β2m) in the ER.

   • Tapasin aligns MHC I with TAP (the peptide loading complex); Calreticulin/Calnexin act as chaperones.

4. Peptide Loading

   • Peptides from TAP bind to MHC I groove.

   • If peptides fit, MHC I remains stable; otherwise, it is rejected.

5. Transport to Cell Surface

   • The MHC I-peptide complex exits ER, passing through Golgi and reaching the cell surface.

6. Recognition by CD8+ T cells

   • CD8+ T cells recognize peptide-MHC I complexes.

   • Foreign peptides prompt CTLs to kill cells while self-peptides are ignored (tolerance).

Clinical Pearl

• Some viruses block the TAP pathway, preventing peptide presentation by MHC I leading to unrecognized infected cells, rendering them suspicious to NK cells.

FLASHCARD FORMAT

• Front: "What transports peptides into the ER for MHC I loading?"

• Back: "TAP (Transporter Associated with antigen Processing)"

• Front: "What degrades cytoplasmic proteins into peptides for MHC I?"

• Back: "Proteasome"

MHC CLASS II PEPTIDE LOADING PATHWAY

Process Flow (Exogenous Pathway)

1. Antigen Uptake

   • APC (dendritic cell, macrophage, or B cell) internalizes extracellular antigen.

2. Antigen Processing in Endosome

   • Antigen in the vesicle is converted to endosome then lysosome; proteases activated under acidic pH (~pH 5), cleaving proteins to peptides (13-25 amino acids).

3. MHC II Assembly in ER

   • MHC II (α + β chains) produced in ER; the Invariant chain (Ii) binds to MHC II and prevents self-peptides from binding.

4. MHC II Transported to Endosome

   • MHC II-Ii complex is carried to late endosome where processed antigens are available.

5. Invariant Chain Cleavage

   • Most of Ii removed by proteases, leaving CLIP in the groove as placeholder.

6. HLA-DM Facilitates Peptide Exchange

   • HLA-DM removes CLIP, allowing binding of antigenic peptides.

   • HLA-DM functions as a peptide editor ensuring quality of peptides.

7. Transport to Cell Surface

   • MHC II-peptide complex moves to surface where it can interact with CD4+ T cells.

8. Recognition by CD4+ T cells

   • If CD4+ T cells identify the specific antigen, they become activated and release cytokines, leading to help for other immune cells.

Clinical Pearl

• Bare Lymphocyte Syndrome Type II results from MHC II deficiency, resulting in failure to activate CD4 T cells and severe immunodeficiency.

FLASHCARD FORMAT

• Front: "What protein blocks the MHC II groove until the complex reaches the endosome?"

• Back: "Invariant chain (Ii) / CLIP"

• Front: "What protein removes CLIP and allows antigenic peptides to bind MHC II?"

• Back: "HLA-DM"

T CELL DEVELOPMENT AND MATURATION

• Where T Cells Develop: In the thymus.

The Journey of T Cell Development

1. Pre-thymic Cells Arrival

   • Hematopoietic stem cells migrate from bone marrow to the thymus as double-negative (DN) thymocytes (without CD4 or CD8).

2. Thymic Cortex - TCR Rearrangement

   • DN cells undergo rearrangement of TCR genes, starting with the β chain followed by the α chain.

   • V(D)J recombination generates diversity; if successful, cells express pre-TCR and become double-positive (DP) thymocytes.

3. Positive Selection (Cortex)

   • Evaluates if TCR can recognize MHC.

   • If the TCR binds MHC even weakly, the cell survives; if not, it undergoes apoptosis. Approx. 95% of thymocytes perish at this stage.

4. Negative Selection (Medulla)

   • Checks if TCR binds self-antigens too strongly. Medullary epithelial cells present self-antigens; those with strong binding undergo apoptosis (autoreactive). Weak or no binding leads to survival, establishing central tolerance.

5. Lineage Commitment

   • Surviving cells lose either CD4 or CD8 depending on TCR preference for MHC II (CD4+) or MHC I (CD8+). They become single-positive (SP) thymocytes.

6. Exit to Periphery

   • Mature, naive T cells exit the thymus, entering the bloodstream and migrating to secondary lymphoid organs (lymph nodes, spleen) to await specific antigen.

Key Concepts

• Positive selection: Tests the ability to recognize MHC (survival if yes).

• Negative selection: Tests reactivity to self-antigens (survival if no).

• Approximately 2-3% of thymocytes survive to mature T cells.

FLASHCARD FORMAT

• Front: "What is positive selection testing for in the thymus?"

• Back: "Can the TCR recognize MHC? If yes, survive."

• Front: "What is negative selection testing for in the thymus?"

• Back: "Does the TCR bind self-antigens too strongly? If yes, die (prevents autoimmunity)."

T CELL RECEPTOR (TCR) DIVERSITY - V(D)J RECOMBINATION

Diversity Generation

• Problem: Need to generate recognition for millions of different antigens while having limited genomic genes (~25,000).

• Solution: Adaptable gene rearrangement during T cell development via V(D)J Recombination.

TCR Structure

• Comprises two chains: α (alpha) and β (beta).

Gene Rearrangement for β Chain

1. Germline DNA includes:

   • V segments (Variable) - ~50 options.

   • D segments (Diversity) - ~2 options.

   • J segments (Joining) - ~13 options.

   • C segment (Constant) - 2 options.

2. Rearrangement Process: RAG1 and RAG2 enzymes (Recombination Activating Genes) facilitate gene cuts.

   • A D segment joins one J segment (choice made randomly).

   • One V segment links to the DJ combination (random selection).

   • Produces functional β chain (VDJ-C).

Gene Rearrangement for α Chain

• Similar methodology but without a D segment.

• Includes V segments (~50) + J segments (~60) → forms VJ-C.

Sources of Diversity

• Combinatorial diversity: Ranges of options from V, D, and J segments.

• Junctional diversity: Imperfect joining at regions adds or deletes nucleotides.

• N-nucleotide addition: TdT enzyme adds random nucleotides at junction sites.

• P-nucleotide addition: Sequences are palindromic at junctions.

• Pairing diversity: Any α can connect with any β chain.

• Result: Over 10^{15} possible TCR combinations!

Clinical Correlations

• Severe Combined Immunodeficiency (SCID): Can arise from RAG1/RAG2 deficiency (lack of TCR/BCR rearrangement leads to absent T and B cells) or from Artemis deficiency (inability to repair DNA breaks during V(D)J). Also from ADA deficiency (toxic metabolites kill developing lymphocytes).

FLASHCARD FORMAT

• Front: "What enzymes perform V(D)J recombination?"

• Back: "RAG1 and RAG2"

• Front: "What disease results from RAG1/RAG2 deficiency?"

• Back: "SCID (Severe Combined Immunodeficiency) - no T or B cells"

B CELL DEVELOPMENT AND MATURATION

• Where B Cells Develop: In the bone marrow.

The Journey of B Cell Development

1. Pro-B Cell Stage

   • Hematopoietic stem cell commits to B cell lineage, expressing CD34 (stem cell marker) and begins to rearrange heavy chain gene (IgH).

2. Pre-B Cell Stage

   • Successful heavy chain rearrangement (μ chain) occurs.

   • μ chain pairs with surrogate light chain, forming pre-BCR and expresses CD10 (immature B cell marker).

   • Note: Heavy chain success prompts light chain production.

3. Immature B Cell Stage

   • Light chain rearrangement occurs (κ or λ), resulting in a completed BCR (IgM).

   • IgM is expressed on the cell surface.

4. Central Tolerance Testing in Bone Marrow

   • Strong binding of BCR to self-antigens leads to apoptosis or receptor editing (a chance to rearrange light chain); weak or non-binding allows survival.

5. Mature Naive B Cell

   • Presence of both IgM and IgD on surfaces (same specificity, differing constant regions).

   • Expresses CD19 (all B cells), CD20 (mature B cells) and CD21 (complement receptor).

   • Moves to peripheral tissues, waiting for antigen exposure.

B Cell Receptor (BCR) Structure

• Surface Structure: Membrane-bound antibody (IgM/IgD) including Igα and Igβ (CD79a, CD79b) for signaling. Upon antigen binding, immediate signal transduction occurs leading to B cell activation.

Key Stages Summary

FLASHCARD FORMAT

• Front: "What surface marker is found on pre-B cells and immature B cells?"

• Back: "CD10"

• Front: "What is the pre-BCR composed of?"

• Back: "Heavy chain (μ) + surrogate light chain"

B CELL RECEPTOR (BCR) DIVERSITY - V(D)J RECOMBINATION

Understanding B Cell Diversity

• The principles are similar to those of TCR but focused on antibodies.

Heavy Chain Rearrangement

• Comprises V segments (~40), D segments (~25), and J segments (~6) → forming VDJ then attaching to the constant region (Cμ initially, can alter later).

Light Chain Rearrangement

• For either κ or λ, V segments are coupled with J segments → forming VJ (lacks D segment). κ is attempted first; if unsuccessful, λ is pursued.

Additional Diversity Mechanisms (Post-Activation)

1. Somatic Hypermutation (SHM): Post-antigen exposure in the germinal center, the AID enzyme introduces point mutations in V regions, generating variants with varied affinities. High-affinity variants are selected during affinity maturation.

2. Class Switch Recombination (CSR): Modifies the constant region (Cμ → Cγ, Cα, or Cε) while retaining the V region (same specificity); this process relies on the AID enzyme.

Clinical Correlations

• Hyper-IgM Syndrome: Resulting from CD40L deficiency (X-linked) or AID deficiency (autosomal recessive), it leads to an inability to class switch, persistently producing IgM and recurring infections.

• RAG deficiency leads to an inability for V(D)J recombination, resulting in no T or B cells (SCID).

FLASHCARD FORMAT

• Front: "What enzyme is required for somatic hypermutation and class switching?"

• Back: "AID (Activation-Induced Deaminase)"

• Front: "What disease results from the inability to class switch from IgM?"

• Back: "Hyper-IgM syndrome (CD40L or AID deficiency)"

T CELL ACTIVATION - THE 3 SIGNALS

Requirements for Full T Cell Activation

Signal 1: Antigen Recognition (SPECIFICITY)

• TCR binds peptide-MHC on APC.

• CD4/CD8 co-receptor interactions stabilize this binding.

• For CD4+: TCR + CD4 binds MHC II on APC.

• For CD8+: TCR + CD8 binds MHC I on infected cell.

• Result: Initiates intracellular signaling (via Lck, ZAP-70) but not yet full activation—serves as protection against unwanted activation.

Signal 2: Costimulation (CONFIRMATION)

• CD28 on T cell binds CD80 (B7-1) or CD86 (B7-2) on APC.

• Acts as a verification signal of an actual infection, enabling continued activation.

• Without this signal, T cells may become ANERGIC (nonresponsive), assisting in avoiding autoimmunity.

• Result: Completeness of T cell activation, increasing IL-2 receptor (CD25) expression and initiating IL-2 production.

Signal 3: Cytokine Signals (DIFFERENTIATION)

• Environmental cytokines dictate the type of helper T cell that is to be formed.

• Different cytokines yield different T helper subsets:

  • IL-12 from macrophages drives CD4+ to TH1.

  • IL-4 from mast cells drives CD4+ to TH2.

  • TGF-β + IL-6 guides CD4+ to TH17.

  • TGF-β + IL-2 directs CD4+ to T regulatory cells (Treg).

• For CD8+: IL-2 (originating from CD4+ T cells or self-produced) leads to proliferation/differentiation into cytotoxic T cells.

Summary Table of Signals

Clinical Applications

• CTLA-4 (CD152): Competes with CD28 for binding to CD80/CD86 but delivers an inhibitory signal, acting as a regulatory mechanism for T cell activation.

  • Ipilimumab targets CTLA-4 to enhance anti-tumor immunity.

  • Abatacept (CTLA-4-Ig) inhibits CD80/CD86 to prevent costimulation, useful in autoimmune diseases (ex: rheumatoid arthritis).

FLASHCARD FORMAT

• Front: "What are the 3 signals required for T cell activation?"

• Back: "Signal 1 = TCR-MHC, Signal 2 = CD28-CD80/86 (costimulation), Signal 3 = Cytokines (differentiation)"

• Front: "What happens if a T cell receives Signal 1 without Signal 2?"

• Back: "Anergy (T cell becomes unresponsive)"

T HELPER SUBSETS - TH1, TH2, TH17, TREG

MASTER TABLE - MEMORIZE THIS

Memory Tricks

• TH1: "1 = Inside" (deals with intracellular pathogens); "IFN-Gamma = Go Get 'em!" (macrophage activation).

• TH2: "2 = Outside, regarding worms" (focuses on extracellular parasites); "IgE = I’m gonna sneEze" (indicates allergic responses).

• TH17: "17 = Neutrophils, 7 rhymes with heaven (mucosal surfaces)" (acts against fungi, leads to possible autoimmunity).

• Treg: "Regulatory = Rules" (controls immune response); "Foxp3 = Fox guards the henhouse" (prevents autoimmunity).

FLASHCARD FORMAT

• Front: "What cytokine induces TH1 differentiation?"

• Back: "IL-12"

• Front: "What does TH1 produce and what does it activate?"

• Back: "Produces IFN-γ, activates macrophages to kill intracellular pathogens"

• Front: "What cytokine induces TH2 differentiation?"

• Back: "IL-4"

• Front: "What does TH2 produce and what does it activate?"

• Back: "Produces IL-4, IL-5, IL-13; activates eosinophils and B cells for IgE production in allergies"

• Front: "What cytokines induce TH17 differentiation?"

• Back: "TGF-β + IL-6 (or IL-1, IL-23)"

• Front: "What does TH17 produce and what does it recruit?"

• Back: "Produces IL-17 and IL-22; recruits neutrophils for extracellular bacteria and fungi"

• Front: "What is the transcription factor for Tregs?"

• Back: "Foxp3"

• Front: "What disease results from Foxp3 mutation?"

• Back: "IPEX syndrome - severe autoimmunity"

CD8+ CYTOTOXIC T CELLS - EFFECTOR MECHANISMS

How CD8+ T Cells Eliminate Infected Cells

• Activation Steps: CD8+ T cell identifies viral peptide on MHC I of infected cells, claiming costimulation (CD28-CD80/86) and receiving IL-2 to proliferate and differentiate into Cytotoxic T Lymphocytes (CTL).

Killing Mechanisms

1. Perforin/Granzyme Pathway (Main Mechanism)

   • CTLs engage target cells via TCR-MHC I interactions, releasing granules comprising perforin (forming pores in the target cell) and granzymes (entering through the pores).

   • Granzymes activate caspases within the target, triggering apoptosis.

   • Target cells suffer “clean death” (no inflammation).

2. Fas-FasL Pathway

   • CTLs express FasL (CD178), which binds to the target's Fas receptor (CD95, a death receptor).

   • Fas engagement leads to caspase activation, resulting in apoptosis in target cells.

Purpose of Two Mechanisms

• The perforin/granzyme pathway acts quickly, while the Fas-FasL acts as a backup, also helping regulate immune responses by terminating T cell activity when necessary.

Targeting of CTLs

• CTLs aim at virus-infected cells, tumor cells, and transplanted tissues (recognizing foreign MHC).

Clinical Correlations

• Perforin Deficiency: Leads to Familial Hemophagocytic Lymphohistiocytosis (FHL), where CTLs cannot eliminate targets, resulting in persistent immune activation and potential fatal cytokine storms.

• Fas/FasL Defects: Result in Autoimmune Lymphoproliferative Syndrome (ALPS), where unable to eliminate activated lymphocytes, leading to autoimmune symptoms and lymphocyte buildup.

FLASHCARD FORMAT

• Front: "What are the two main killing mechanisms of CD8+ T cells?"

• Back: "1) Perforin/Granzyme pathway (main), 2) Fas-FasL pathway"

• Front: "What disease results from perforin deficiency?"

• Back: "Familial Hemophagocytic Lymphohistiocytosis (FHL)"

• Front: "What do granzymes do after entering the target cell?"

• Back: "Activate caspases → trigger apoptosis"

ANTIBODY (IMMUNOGLOBULIN) STRUCTURE

Basic Structure

1. Components:

   • 2 Heavy chains (identical).

   • 2 Light chains (identical) - can be κ or λ.

   • Connected by disulfide bonds to form a “Y” shape.

Regions Identified

1. Variable Region (V):

   • Present at the top Y portion (Fab region - Fragment antigen binding).

   • Comprises 1 V region from each chain, invariably forming an antigen-binding site through VH (heavy) + VL (light).

   • Each antibody accommodates 2 antigen-binding sites (bivalent).

   • CDRs (Complementarity-Determining Regions) signify hypervariable loops interacting with antigens – 3 CDRs for each V region; diversity is pronounced here.

2. Constant Region (C):

   • Located at the bottom portion (Fc region - Fragment crystallizable).

   • Determines antibody class (IgG, IgM, IgA, IgE, IgD).

   • Binds to Fc receptors on immune cells fostering immune responses (opsonization)

   • Mediate antibody functions depending on the class.

Hinge Region

• Flexible region permitting antibodies to adjust their binding angles to antigens.

Antibody Fragments When Cleaved

FLASHCARD FORMAT

• Front: "What part of the antibody binds antigen?"

• Back: "Fab region (Variable region - VH + VL)"

• Front: "What part of the antibody determines its class and effector functions?"

• Back: "Fc region (Constant region)"

• Front: "What are CDRs?"

• Back: "Complementarity-Determining Regions - hypervariable loops in V region that directly contact antigens"

ANTIBODY CLASSES - IgG, IgM, IgA, IgE, IgD

MASTER TABLE - MEMORIZE THIS

Detailed Breakdown of IgG

The Workhorse

• Structure: Monomer crossing placenta.

• Subclasses: IgG1 (foremost), IgG2, IgG3, IgG4.

• Functions: Opsonization (covalently preparing pathogens for phagocytosis; most versatile antibody), complement activation (classical pathway activation by binding to Fc), antibody-dependent cell-mediated cytotoxicity (ADCC), and neutralization (blocking abilities of toxins/viruses).

• Clinical Note: IgG is the major antibody during the secondary immune response and the sole antibody crossing the placenta, thus safeguarding newborns; prevalent in conditions like hemolytic disease if maternal IgG targets fetal RBCs.

Detailed Breakdown of IgM

The First Responder

• Structure: Pentamer; largest antibody.

• Functions: Primary antibody produced during the initial immune response, outstanding at complement activation—facilitates numerous C1q engagements.

• Clinical Note: Elevated levels indicate acute infections, typical in conditions leading to Waldenström macroglobulinemia and as natural antibodies (against ABO blood types).

Detailed Breakdown of IgA

The Mucosal Guardian

• Structure: Monomer (serum) or dimer (secretory) plus J chain; found in mucosal surfaces.

• Functions: Prevents pathogen adherence, neutralizes toxins, and serves immune exclusion.

• Clinical Note: IgA deficiency is the most common immunity-related immunodeficiency.

Detailed Breakdown of IgE

The Allergy Antibody

• Structure: Monomer found attached to mast cells/basophils via Fc receptors.

• Functions: Key player in IgE-mediated allergic reactions; tackles antiparasitic immunity.

• Clinical Note: Elevated in allergies and parasitic diseases; involved in conditions requiring anti-IgE therapies like Omalizumab for severe asthma.

Detailed Breakdown of IgD

The Mystery

• Structure: Monomer found on naïve B cells' surfaces alongside IgM.

• Function: Acts as the B cell receptor involved in B cell activation, with its specific roles still being elucidated.

FLASHCARD FORMAT

• Front: "Which antibody is the first to be produced in a primary immune response?"

• Back: "IgM"

• Front: "Which antibody is best at activating complement?"

• Back: "IgM (pentamer structure)"

• Front: "Which is the most abundant antibody in serum?"

• Back: "IgG (75%)"

• Front: "Which antibody crosses the placenta?"

• Back: "IgG (only one)"

• Front: "Which antibody is found in mucous membranes, tears, saliva, and breast milk?"

• Back: "IgA"

• Front: "What is the structure of secretory IgA?"

• Back: "Dimer + J chain + secretory component"

• Front: "Which antibody is responsible for Type I hypersensitivity (allergies)?"

• Back: "IgE"

• Front: "Where is IgE normally found?"

• Back: "Bound to mast cells and basophils"

• Front: "Which antibody has the longest half-life?"

• Back: "IgG (21 days)"

• Front: "Which antibody is found on the surface of naive B cells?"

• Back: "IgM and IgD"

B CELL ACTIVATION - T-DEPENDENT vs T-INDEPENDENT

Two Pathways for B Cell Activation

1. T-Dependent B Cell Activation (Requires T Cell Assistance)

   • Location: Secondary lymphoid organs (lymph node, spleen).

   • Steps:

   1. Antigen recognition by BCR (IgM or IgD) occurs, leading to endocytosis and processing of antigen for MHC II presentation.

   2. Activated B cells migrate to the T cell zones, presenting peptides on MHC II, which are recognized by CD4+ T helper cells (already activated by dendritic cells).

   3. CD40L on T cell binds CD40 on B cell, along with cytokine release, fully activating B cells.

   4. Fully activated B cells can proliferate, forming germinal centers in lymph node follicles, where somatic hypermutation, affinity maturation, and class switching occur.

   5. Resulting in differentiation into Plasma cells (memory) or Plasma cells (antibody-producing factories).

2. T-Independent B Cell Activation (No T Helper Cell Needed)

   • Types:

     • Types include Type 1 (TI-1): Polythingal activation via mitogenic substances activating ALL B cells indiscriminately, and Type 2 (TI-2): Repetitive epitopes that cause BCR cross-linking through polysaccharide capsular structure or flagellin to yield a robust response.

   • Characteristics:

     • This process leads to restricted output in antibodies produced (mainly IgM without class switching or somatic hypermutation leading to a weaker memory response).

   • Note: Important in defending against encapsulated bacteria; young children (<2) have weak T-independent responses (pneumococcal vaccines are less effective in infants).

   • Result: Produces mainly IgM with minimal memory capabilities.

Summary Table of B Cell Activation

Clinical Correlations

• Hyper-IgM Syndrome: Stemming from CD40L (X-linked) or AID deficiency (autosomal recessive) prevents class switching—consequently resulting in increased IgM production with recurrent bacterial infections.

• Bruton's Agammaglobulinemia: BTK (Bruton's tyrosine kinase) deficiency leads to the absence of mature B cells and very low antibodies, leading to infections after 6 months of age when maternal antibodies dwindle.

• Common Variable Immunodeficiency (CVID): A defect in B cell differentiation leading to low IgG/IgA levels correlating to recurrent infections and autoimmune issues.

FLASHCARD FORMAT

• Front: "What type of antigens require T cell help for B cell activation?"

• Back: "Protein antigens (T-dependent activation)"

• Front: "What signal does the T helper cell provide to B cells for full activation?"

• Back: "CD40L (on T cell) binds CD40 (on B cell) + cytokines"

• Front: "What happens in the germinal center during B cell activation?"

• Back: "Somatic hypermutation (affinity maturation) and class switch recombination"

• Front: "What type of antigens can activate B cells without T cell help?"

• Back: "Polysaccharides with repetitive epitopes (T-independent type 2)"

• Front: "What disease results from CD40L deficiency?"

• Back: "Hyper-IgM syndrome (can’t class switch, stuck making IgM)"

• Front: "What enzyme is needed for class switching and somatic hypermutation?"

• Back: "AID (Activation-Induced Deaminase)"

ANTIBODY FUNCTIONS - MECHANISMS TO ELIMINATE PATHOGENS

Seven Main Mechanisms

1. Neutralization

   • Description: Antibody binds the pathogen or toxin, inhibiting their ability to attach to host cells.

   • Examples: IgG/IgM neutralizing viruses, or toxins preventing receptor binding (e.g., botulinum).

   • Clinical Insight: Passive immunity (IVIG, antitoxins) can be applied for swift neutralization effects against toxins or pathogens, especially during acute care.

2. Opsonization

   • Description: Antibody coats the pathogen, signaling phagocytes via their Fc receptors to facilitate uptake/eating of the pathogen (macrophages or neutrophils).

   • Mechanism: IgG binds to bacterial surface antigens allowing phagocytes (via FcγR [Fc gamma receptor]) to engulf and destroy the microbes.

   • Clinical Insight: Opsonization is pivotal in combatting bacterial threats especially when patients have deficiencies in IgG resulting in recurrent infections.

3. Complement Activation

   • Description: Antigen-antibody complexes initiate complement activation leading to inflammation, opsonization, and lysis of the target.

   • Mechanism: Binding of IgM/IgG (especially IgM) activates classical complement pathways, yielding C3a, C5a, and membrane attack complexes.

   • Clinical Insight: Essential for combating encapsulated bacteria (e.g., Neisseria, Streptococcus pneumoniae).

4. Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

   • Description: An antibody attaches to a target cell marking it for destruction by NK cells or other cells harboring Fc receptors.

   • Mechanism: Primarily IgG directs these responses via CD16 (FcγRIII) connections on NK cells leading to cell death through perforin/granzyme pathways.

   • Clinical Insight: Rituximab and trastuzumab work by leveraging ADCC mechanisms to target specific malignancies.

5. Agglutination

   • Description: Multiple pathogens get aggregated through cross-linking due to antibodies, facilitating their clearance by phagocytes.

   • Mechanism: Antibodies with multiple binding sites (IgG, best being IgM) alter pathogen distribution, allowing easier phagocytosis.

   • Clinical Insight: Utilized in blood typing and laboratory diagnostics.

6. Mast Cell Degranulation (Type I Hypersensitivity)

   • Description: Antigen-bridged IgE bound to mast cells triggers the release of inflammatory mediators (like histamine).

   • Mechanism: Re-exposure to allergens leads to mast cell activation: histamine release instigates symptoms characteristic of allergies.

   • Clinical Insight: Treatments for severe reactions often include medications that target these pathways (EpiPen containing epinephrine).

7. Mucosal Immunity (IgA)

   • Description: IgA present in mucosal surfaces prevents pathogen attachment enabling their clearance via peristalsis.

   • Mechanism: Secreted IgA in mucus traps pathogens, enable clearance without triggering an immune response.

   • Clinical Insight: Breastfeeding offers newborns a protective measure against gastrointestinal pathogens via IgA in colostrum.

Summary Table of Antibody Functions

FLASHCARD FORMAT

• Front: "What is opsonization?"

• Back: "Antibody coats pathogen → Fc binds phagocyte Fc receptors → enhanced phagocytosis"

• Front: "Which antibody is best at activating complement?"

• Back: "IgM (pentamer efficiently binds C1q)"

• Front: "What is ADCC?"

• Back: "Antibody-Dependent Cell-mediated Cytotoxicity - IgG-coated target killed by NK cells via CD16"

• Front: "What causes mast cell degranulation in allergies?"

• Back: "Allergen cross-links IgE bound to mast cell FcεRI receptors"

• Front: "How does secretory IgA protect mucosal surfaces?"

• Back: "Binds pathogens in mucus lumen, prevents attachment to epithelial cells (immune exclusion)"

CLINICAL SCENARIO: "STEP ON A RUSTY NAIL"

Synopsis: Immune Response to a Wound

PHASE 1: IMMEDIATE (Minutes) - INNATE IMMUNITY

1. Physical Barrier Breached

   • Skin integrity compromised; bacteria enter.

2. Innate Immune Cells Detect Pathogens

   • Macrophages recognize bacteria through TLRs (TLR4 recognizes LPS, TLR5 for flagellin).

   • Mast cells degranulate, releasing histamines, causing vasodilation/increased permeability.

3. Inflammation

   • Cytokines (IL-1, IL-6, TNF-α) activate endothelial cells, eliciting recruitment signals.

4. Complement Activation

   • The alternative pathway engages on bacterial surfaces; C3b coordinates opsonization with C5a acting as a neutrophil chemotactic signal.

5. Neutrophils Mobilize

   • Detect IL-8 and C5a for rolling, activation, arrest, and transmigration, leading to phagocytosis of bacteria.

6. Macrophages Arrive

   • Monocytes migrate to tissue; differentiate into macrophages for additional phagocytosis and antigen presentation.

   • Result: Majority of bacteria are eliminated through innate immunity mechanisms.

PHASE 2: DAYS 3-7 - ADAPTIVE IMMUNITY PRIMING

1. Dendritic Cell Activity

   • Antigen capture by dendritic cells leads to phagocytosis and MHC II presentation.

   • Dendritic cells mature and transport to lymph nodes.

2. T Cell Activation

   • Naïve CD4+ T Cells in lymph nodes activate upon recognizing presented bacterial peptides via 3 signals (TCR-MHC II, CD28-CD80, and IL-12 signaling).

   • CD4+ T cells differentiate into TH1 (due to IL-12) producing IFN-γ to activate macrophages.

3. B Cell Activation

   • B cells present antigens processed by BCR on MHC II to activated TH1 cells and fully activate after CD40-CD40L interaction and cytokine stimulation.

4. Germinal Centers Formation

   • Activated B cells transition into germinal centers for affinity maturation via somatic hypermutation, with class switching from IgM to IgG occurring (driven by cytokines).

5. CD8+ T Cells Activation

   • If bacteria dwell within host cells, CD8+ cells recognize bacterial peptides on MHC I, proliferating into cytotoxic T lymphocytes (CTLs).

PHASE 3: WEEKS 1-2 - ADAPTIVE EFFECTOR PHASE

1. Antibodies at Wound Site

   • IgG reaches the affected area due to plasma cell activity facilitating opsonization and neutralization.

2. TH1 Assistance

   • IFN-γ production by TH1 cells enhances macrophage activity, allowing for the elimination of intracellular bacteria.

3. CD8+ CTLs Response

   • Target infected cells presenting bacterial peptides, employing perforin/granzyme mechanisms to eliminate infected cells.

4. Resolution of Inflammation

   • Post-clearance, Tregs inhibit unnecessary immune responses, leading to macrophages transitioning to M2 for tissue repair.

PHASE 4: MEMORY (Lifelong)

1. Memory Cell Retention

   • Memory B cells remain in lymphoid structures; T cell memory resides in circulation and tissues.

2. Secondary Response

   • Subsequent exposure to the same pathogen triggers a prompt and robust immune response facilitated by memory cells, characterized by rapid antibody generation (days) and heightened efficacy (high-affinity IgG from prior exposures).

3. Vaccination Principles

   • Utilizing inactivated (toxoid) or attenuated pathogens activating complete immune responses while avoiding disease onset, thus forming memory B and T cells inducing rapid responses upon re-exposure.

Summary of the Immune Journey (Rusty Nail)

FLASHCARD FORMAT

• Front: "How does innate immunity 'talk to' adaptive immunity?"

• Back: "Dendritic cells: capture antigens at the site of infection → travel to lymph nodes → present on MHC II → activate naive T cells"

• Front: "Why is the secondary immune response faster than primary?"

• Back: "Memory B and T cells already exist → rapid antibody production (days vs weeks)"

• Front: "What type of antibody dominates in the secondary response?"

• Back: "IgG (high-affinity, already class-switched from primary response)"

• Front: "How does vaccination create immunity without causing disease?"

• Back: "Uses inactivated or weakened pathogen → triggers adaptive response → creates memory cells → rapid response upon actual exposure"

IMMUNODEFICIENCY DISEASES - Quick Reference

T Cell Deficiencies

B Cell Deficiencies

Combined Deficiencies

Phagocyte Defects

(From Previous Guide)

Complement Deficiencies

(From Previous Guide)

VACCINES - Types and Mechanisms

Types of Vaccines

Why Conjugate Vaccines Work Better in Children

• Polysaccharide alone (like Pneumovax) induces T-independent mechanic leading to minimal memory formation.

• The Conjugate Vaccine (e.g., PCV13) binds polysaccharide to protein enabling CD4+ T cell activation and profound memory outcomes via T-dependent pathway capabilities.

HYPERSENSITIVITY REACTIONS - Quick Reference

Types of Hypersensitivity

Memory Trick for Hypersensitivity

• ACID: Anaphylactic, Cytotoxic, Imune complex, Delayed.

FINAL REMINDERS FOR TOMORROW

Important Topics to Review

• CD10: Marker for immature B cells (emphasized by the professor).

• CD19: The most critical B cell marker for all B cells.

• CD markers: Knowledge about all key markers for identification of cell types.

• MHC I and MHC II: Structure, function, and mechanisms behind peptide loading must be understood.

• Cell-mediated immunity: T cell activation, differentiation, and effector functions should be well-grounded.

• Humoral immunity: Comprehension of B cell activation and antibody production is critical.

• Clinical Applications: Understand the rusty nail scenario impacts, responses, and immune mechanisms.

High-Yield Facts for Review

• CD Markers: CD3 = T cells, CD4 = helper, CD8 = cytotoxic; CD10 = immature B, CD19 = all B, CD20 = mature B.

• Costimulation: CD28 = CD80/CD86 significance, CD40 = helper cell roles.

• MHC: MHC I presence in all nucleated cells presenting internal items to CD8; while MHC II is limited to APCs presenting external items to CD4.

• T Helper Functions: TH1 = IL-12 driving intracellular defense via IFN-γ release; TH2 = IL-4 focusing on external issues amid immunity; TH17 focusing on extracellular threats while Treg manages overall immune filtering.

• For antibodies: IgM = first response; IgG = major contributor with important functions across the board; IgA = crucial in mucosal immunity; IgE = allergy-linked; and IgD = lesser-known functionally.

• B Cell Activation Pathways: Remember T-dependent versus T-independent activated mechanisms and outcomes.

NOTE: You now possess extensive details on all crucial elements relating to Adaptive Immunity.

Prepare adequately, and you will succeed in your upcoming assessments! 🔥💪